1. Field of the Invention
The present invention relates to a liquid crystal display device used for a display portion of an electronics device and a method of manufacturing the same.
2. Description of the Related Art
A liquid crystal display device generally has two substrates each provided with a transparent electrode and a liquid crystal layer interposed between both of the substrates. In a liquid crystal display device, a predetermined voltage is applied between the transparent electrodes to drive a liquid crystal and the light transmittance is controlled for each of, pixels, and thereby a desired display is obtained. In recent years, a market of the liquid crystal display devices is expanding and there are various demands for the liquid crystal display devices. Among these, a demand for improvement in display quality is particularly strong. In order to obtain excellent display without brightness unevenness, a technology in which pillar spacers are formed on a substrate to uniformly control a thickness (cell gap) of a liquid crystal layer is becoming a mainstream.
Existing pillar spacers are disposed in a predetermined arrangement pattern in a substrate surface. These pillar spacers support a gap between a thin film transistor (TFT) substrate and an opposite substrate that are attached to each other. As formation materials of the pillar spacers, in general, resinous materials such as acrylic resins and novolac resins are used. However, these resinous materials have, from the physical properties thereof, an elastic deformation region and a plastic deformation region. When a local force is applied from the outside of a panel, the pillar spacers undergo the plastic deformation; that is, even after the force was removed, the pillar spacers do not restore an original height. Accordingly, in a loaded portion where the local force is applied, cell gap unevenness is caused. In order to inhibit the cell gap unevenness from occurring, the number of the pillar spacers may be increased or a pillar area of the pillar spacer may be increased to make the liquid crystal panel harder. However, when a density of the pillar spacers is made excessively high, relative to a volume decrease of liquid crystal under low temperatures, the pillar spacer prevents accompanying deformation, a vacuum region is generated, resulting in a problem that bubbles are generated there. Accordingly, the arrangement density of the pillar spacers cannot be made so high; that is, there is an upper limit on the arrangement density. In the pillar spacers at the arrangement density under this restriction, the resistance of the cell gap against an external local pressurization cannot be sufficiently obtained.
In order to solve the problem, patent document 1 discloses a technology in which two kinds of pillar spacers having different heights are formed. Furthermore, patent document 2 discloses a technology in which pillar spacers are disposed in two or more kinds of arrangement patterns, and by making use of steps of film thickness on the TFT substrate side, when substrates are attached, two kinds of pillar spacers practically different in height are formed. The liquid crystal display devices described in patent documents 1 and 2 include first pillar spacers that sustain the cell gap during normal and low temperatures and second pillar spacers that sustain the cell gap when a local pressure is externally applied, and thereby these have a structure that does not cause bubbles at low temperatures and can stand even the external local pressurization.
Now, a method of manufacturing the above liquid crystal display device will be explained. In FIGS. 22A through 22E, existing manufacturing steps of a liquid crystal display device are shown. In FIGS. 23A through 23D, states in the respective steps of two kinds of pillar spacers having different heights are schematically shown. Firstly, as shown in FIGS. 22A and 23A, on an outer periphery of one substrate 104 on which two kinds of pillar spacers 140 and 142 different in height are formed, a sealing material 120 is coated so that part thereof is opened as a liquid crystal injection port 122.
In the next place, as shown in FIG. 22B, one substrate 104 and the other substrate 102 are attached through the sealing material 120, and thereby a vacant cell 108 is formed. At this time, as shown in FIG. 23B, a first pillar spacer 140 higher in the height comes into contact with the other substrate 102 and a second pillar spacer 142 lower in the height does not come into contact with the substrate 102.
Subsequently, the vacant cell 108 is transferred into a not shown liquid crystal injector followed by evacuating the inside of the liquid crystal injector. Thereafter, the liquid crystal injection port 122 of the vacant cell 108 is brought into contact with liquid crystal 106 filled in a liquid crystal dish 124 followed by releasing the inside of the liquid crystal injector to air. Thereby, as shown in FIGS. 22C and 23C, the liquid crystal 106 is injected into the vacant cell 108.
After the vacant cell 108 filled by the liquid crystal 106 is taken out of the liquid crystal injector, as shown in FIG. 22D, both substrates 102 and 104 are pressurized under a definite pressure (shown with a bold arrow mark in the drawing), and thereby excessive liquid crystal 106 is extruded and thereby a cell gap is controlled. At this time, as shown in FIG. 23D, the substrate 102 comes relatively nearer by a definite distance to the substrate 104 from a position shown with a broken line in the drawing. Thereby, the first pillar spacer 140 is compressed by a definite displacement amount. The displacement amount is controlled so as to be smaller than difference of heights between the first and second pillar spacers 140 and 142; accordingly, the second pillar spacer 142 does not come into contact with the substrate 102. Thereafter, the liquid crystal injection port 122 is sealed with a sealant 126, and thereby a liquid crystal panel 110 is prepared.
Thus, according to the existing liquid crystal injection method (vacuum injection method), a definite pressure is applied between both substrates 102 and 104 and excessive liquid crystal 106 is discharged from the liquid crystal injection port 122. Accordingly, an internal pressure of the liquid crystal panel 110 is controlled constant and the compression displacement amount of the first pillar spacer 140 becomes constant. The prepared liquid crystal display device includes the first pillar spacers 140 that sustain the cell gap during normal temperatures and low temperatures and the second pillar spacers 142 that sustain the cell gap when the local pressure is applied; accordingly, bubbles are not generated at low temperatures and the external local pressure can be withstood.
Recently, as means for realizing shortening of liquid crystal injection time, a liquid crystal dropping (ODF; One Drop Filling) method where substrates attaching and liquid crystal injection are simultaneously carried out is becoming popular. In FIGS. 24A through 24C, manufacturing steps of a liquid crystal display device in which the ODF method is applied are shown. In FIGS. 25A and 25B, states of two kinds of pillar spacers 140 and 142 different in the height are schematically shown for the respective steps.
Firstly, as shown in FIGS. 24A and 25A, on the whole circumference of an outer periphery of one substrate 104, a sealing material 120 is coated in seamless manners. Subsequently, as shown in FIGS. 24B and 25B, a predetermined amount of liquid crystal 106 is dropped on the substrate 104 (or the other substrate 102) by use of a dispenser. Still subsequently, as shown in FIG. 24C, both substrates 102 and 104 are attached in a vacuum followed by returning to an atmospheric pressure, and thereby the liquid crystal 106 is injected. At this time, the cell gap is controlled by a dropping amount of the liquid crystal 106, and the first pillar spacers 104 are compressed by a predetermined displacement amount.
The first and second pillar spacers 140 and 142 are formed with a certain degree (substantially 0.1 to 0.2 μm) of dispersion between the heights thereof. Accordingly, when a definite amount of the liquid crystal 106 is always dropped, depending on the heights of the first pillar spacers 140, an internal pressure of the liquid crystal 106 varies. Accordingly, after the substrates are attached, the dispersion is also caused in the compression displacement amounts of the first pillar spacers 140.
FIGS. 26A, 26B, 27A and 27B show states before and after the attaching of the substrates of a liquid crystal display device that is manufactured by use of the ODF method. FIGS. 26A and 27A show the pillar spacers 140 and 142 before the attaching of the substrates, and FIGS. 26B and 27B show the pillar spacers 140 and 142 after the attaching of the substrates. As show in FIGS. 26A and 26B, in the liquid crystal display device, a cell gap G that is determined by a dropping amount of the liquid crystal 106, a height H1 of the first pillar spacer 140 and a height H2 of the second pillar spacer 142 satisfy G<H1 and G<H2. Accordingly, after the attaching of the substrates, both the first and second pillar spacers 140 and 142 sustain the cell gap G and the compression displacement amounts of the first and second pillar spacers 140 and 142 become larger; accordingly, the internal pressure becomes lower. As a result, the first and second pillar spacers 140 and 142 prevent deformation following a decrease in volume of the liquid crystal 106 at low temperatures, resulting in foaming at low temperatures.
On the other hand, in a liquid crystal display device shown in FIGS. 27A and 27B, a cell gap G that is determined by a dropping amount of the liquid crystal 106, a height H1 of the first pillar spacer 140 and a height H2 of the second pillar spacer 142 satisfy G>H1 and G>H2. Accordingly, after the attaching of the substrates, the first and second pillar spacers 140 and 142, not coming into contact with the other substrate 102, are not compressed, and a higher internal pressure results. Accordingly, gravity unevenness is caused and the resistance against the external local pressure cannot be obtained.
Thus there is a problem in that a liquid crystal display device that does not cause foaming at low temperatures, can resist against the external local pressurization and can exhibit excellent display quality cannot be manufactured according to the ODF method.
[Patent document 1] JP-A-2001-201750
[Patent document 2] JP-A-2003-156750
[Patent document 3] JP-A-2002-202512